1. Field of the Invention
The present invention relates to a connected image forming system for performing image forming by connecting a plurality of image forming apparatuses.
2. Description of the Related Art
In recent years, demands for high speed, high image quality, color print, low energy consumption, and the like have become increasingly high for image forming apparatuses such as copying machines, printers, and multifunctional peripherals. For such electrophotographic image forming apparatuses, a tandem image forming apparatus has been proposed in which relay units that convey a printing material are provided between a plurality of image forming apparatuses to perform image forming. In this type of image forming apparatuses, each apparatus has charging, exposure, developing, transfer, and fixing devices. A first image forming apparatus forms an image on one side (first side) of a printing material, and inverts the printing material. Then, a next image forming apparatus forms an image on the other side (second side) of the printing material. There has been proposed a technique for doubling the number of output pages of double-sided printing as compared with the number of output pages of one conventional image forming apparatus by sharing an image forming operation, thereby achieving higher speed.
By connecting, using relay units, a plurality of image forming apparatuses each of which is usable by itself, it is possible to readily commercially introduce such connected image forming apparatuses without newly designing hardware. Unlike a scheme of making use of image forming apparatuses by connecting them in parallel, such connected image forming apparatuses can include a large-capacity sheet feed deck, post-processing device, and the like upstream or downstream, thereby advantageously sharing peripheral applications, facilitating inline processing, saving space, and reducing the introduction cost.
According to, for example, U.S. Pat. No. 4,579,446, connected image forming apparatuses are characterized by connecting two image forming apparatuses in series to have an inversion unit therebetween, and cause the apparatuses to respectively print front and back sides, thereby implementing double-sided printing.
The above connected image forming apparatuses have the following problem. That is, in U.S. Pat. No. 4,579,446, if one of the two apparatuses connected in series enters a state that is disabling for image forming such as a toner shortage state, it becomes impossible to continue an image forming operation. Therefore when only one of the apparatuses stops, a printing operation itself also stops. In addition to a toner shortage state, when printing materials run out, an image forming operation stops.
Connected image forming apparatuses which are used by connecting two image forming apparatuses have been mainly desired in the POD market such as a quick printing field requiring productivity. In such a market, a large amount of output and continuous operation is needed. Therefore, for example, it is required that even if an operator were to go home during a 24-hour operation, it should be possible to have continuous operation unattended until the next morning.
In connected image forming apparatuses, when one main body of the apparatuses enters an image forming disable state such as a toner or printing material shortage state, it is possible to switch control to continue a printing operation using the other normal main body, thereby continuing the printing operation. By performing the above control, however, one image forming apparatus stops image forming and the other image forming apparatus continues image forming. When the image forming apparatus which stopped image forming recovers an image forming enable state by maintenance processing performed by an operator, the productivity may decrease depending on the timing when the double-sided image forming operation by two apparatuses resumes.